V. 



Anlagen. 



WHEN one is studying the actual development of any organism, 

 one sees, in microscopic sections, the beginnings of organs before 

 these have assumed definite shape and form. In some cases, as in 

 the case of the developing reproductive organs of many animals, these 

 beginnings come into visibility first as three or four definite cells, 

 clearly marked out from the cell-mass in which they lie, by difference 

 in character which is directly visible, or by difference in character 

 apparent only by difference of reaction to certain stains. In other 

 cases, perhaps in the majority of cases, the beginning of an organ or 

 structure, so far as visibility to us goes, is not two or three separate cells, 

 but a thickening of tissue resulting from the more active proliferation 

 or multiplication of cells in some definite area. For instance, in 

 many vertebrates the nervous system, as it first becomes visible under 

 the microscope, is a broad band or plate distinguishable under the 

 microscope by the relative opacity it causes along the dorsal surface 

 of the embryo. At first the whole young embryo of an animal or 

 plant is, in the majority of cases, a mass of cells indistinguishable 

 from each other in appearance. At various points in this mass the 

 various organs come up into visibility, successively or simul- 

 taneously, in definitely localised regions of the common mass of 

 germinal cells. 



For these first visible appearances of organs German writers use 

 the word " Anlage," borrowing it from its familiar use as the word 

 denoting the actual concrete beginning of some operation. Thus die 

 Anlage eines Gartens zu niachen is not to draw out the plan of a garden 

 on paper, but to make the footpaths and flower-beds. 



Starting from the objective and visible fact, that organs appear 

 as visible proliferations or buds of cells in definite regions, biologists 

 in many cases have carried the idea back to the invisible and 

 theoretical. Some have supposed that the hereditary mass of the 

 fertilised egg-cell itself is a mosaic of definitely-placed though 

 invisible primitive beginnings of organs, and that the visible 

 beginnings of these organs, in the embryonic mass that results from the 

 division of the egg-cell, are due to the presence in such areas of the 



